1. Field of the Invention
This invention relates to telephone systems, and more particularly to central-office equipment for Digital Subscriber Lines (DSL) that use frequency-division-multiplexing to share the line with Plain Old Telephone Service (POTS) equipment.
2. Description of the Related Art
The demand for telephone bandwidth continues to increase as more telephone customers send data traffic over phone lines. While it is feasible to run high-speed fiber-optic cable to some new customers, existing customers are connected to the phone system by slower coppers wires. The cost of replacing existing copper wires with higher-speed fiber-optic cable is prohibitive. Thus, higher-bandwidth technologies that use the existing copper-cable phone lines are desirable.
Integrated Services Digital Network (ISDN) boosted data rates over existing copper phone lines to 128 kbps. Special termination and conditioning of the existing copper phone lines is required for ISDN. ISDN's future is in doubt now that newer analog modems are reaching 56 kbps without expensive conditioning of the phone lines. Digital-Subscriber Lines (DSL) are now becoming available. DSL provides bandwidth up to 8 Mbps downstream, or up to 2 Mbps symmetric. DSL approaches the bandwidth of T1 lines, about 1.5 Mbps. Several variations of DSL technology are being explored, such as HDSL, IDSL, SDSL, RADSL and ADSL. ADSL (asymmetric DSL) is particularly attractive for consumer Internet applications where most of the data traffic is downloaded to the customer. Upstream bandwidth for uploading data can be reduced to increase downstream bandwidth since most Internet traffic is downstream traffic. See U.S. Pat. Nos. 5,461,616, 5,534,912, and 5,410,343 for descriptions of ADSL technology.
VOICE AND DATA CALLS ON ADSL--FIG. 1
Some DSLs such as ADSL have the advantage that ordinary voice calls can share the same line with data calls. The lower-frequency band of ADSL is used for voice calls, while the upper frequencies are used for data calls. FIG. 1 is a diagram of the partitioning of frequency bands for ADSL data and voice calls. Plain-old-telephone service (POTS) voice calls are transmitted over low-frequency POTS band 2, as they are for standard telephone lines. POTS band 2 operates from about 100 Hz to 4 kHz. Since this is the same frequency range as standard telephones, ordinary telephone equipment can be used over POTS band 2.
Reverse channel 4 is for uploads from the customer, or for sending commands and user input from the customer to the central office and eventually to the Internet. Some embodiments may use a bi-directional channel in place of reverse channel 4. Reverse channel 4 operates at about 85 to 95 kHz, or up to 1 Mbps.
Wide-band 5 carries the bulk of the ADSL-line bandwidth. Wide-band 5 carries ADSL data to the customer at up to 8 Mbps. Wide-band 5 is a frequency band from 100 to 500 MHz. Thus ADSL is asymmetric; using a wide frequency band for downloads while using a relatively narrow frequency band for uploads. The lowest frequencies are reserved for POTS.
VOICE CALLS ON ADSL LINES WHEN POWER FAILS
Although voice calls can be made over ISDN, special terminal adapters must always be powered. When power fails, ISDN voice calls cannot be made. Thus, customers are told to have a standard POTS line for emergencies, in addition to the ISDN line.
ADSL voice calls can be made even when the power fails. External power is not required for the frequency-splitter circuit that separates the low-frequency voice band from the high-frequency data band. The splitter can be made of entirely passive components that do not require external power, and the power provided by the central office over the phone line is sufficient to power the POTS telephone. Since central offices have back-up power generators to power ordinary POTS lines during emergencies and power failures, ADSL lines, like POTS lines, continue to carry voice calls during power failures. Thus, a single ADSL line can serve as an emergency voice line. A second POTS line is not required as is true for ISDN.
ADSL EQUIPMENT INCLUDES FREQUENCY SPLITTER
Like ISDN, ADSL requires some line conditioning of the copper phone lines. Special equipment is needed at both the customer premises and at the phone company's central office where the customer's copper phone line ends. FIG. 2 is a diagram of a prior-art ADSL phone line highlighting the frequency splitters.
Copper phone line 20 is a pair of copper lines running from central office 8 to the customer. The phone customer has installed customer premises equipment 6. Since ADSL uses high frequencies for data traffic and low frequencies for voice calls, the signal received over copper phone line 20 must be split into high- and low frequency components. Splitter 12 contains a low-pass filter that outputs the low-frequency components from copper phone line 20. These low-frequency components carry the voice calls that are sent to telephone set 10. Telephone set 10 is a standard POTS analog telephone set. Additional phone sets, fax machines, or analog modem equipment can be connected to telephone set 10 as phone-line extensions as is well-known.
Splitter 12 also contains a high-pass filter that outputs the high-frequency components to ADSL modem 14. ADSL modem 14 converts the high-frequency analog signal from splitter 12 into digital data. Computer equipment can read this digital data and transmit digital data to ADSL modem 14 for conversion to analog-voltage modulations. Splitter 12 also mixes high-frequency data from ADSL modem 14 with the low-frequency voice from telephone set 10 and transmits the combined signal over copper phone line 20 to central office 8.
Copper phone line 20 is typically only one or two miles in length. Lines longer than 18,000 feet have too many loses for use as ADSL lines. Central office 8 receives copper phone line 20 and splits off the high-frequency components with splitter 16. The high-frequency components from splitter 16 are sent to ADSL modem 18, which converts the analog-voltage high-frequency signal to a digital data steam. The data stream can then be combined with a high-speed data highway or backbone.
Splitter 16 sends low-frequency components to conventional telephone switch 19, which is similar to other line cards that terminate POTS lines. Conventional telephone switch 19 connects to other switch circuits to be combined with other calls and sent to a pulse-code-modulated (PCM) highway for transmission to other central offices or to the long-distance networks.
Incoming voice calls received by conventional telephone switch 19 are combined by splitter 16 with modulated data traffic from ADSL modem 18. The combined signal is transmitted over copper phone line 20 to customer premises equipment 6.
ANALOG SPLITTER IS BULKY, COSTLY
FIG. 3 is a diagram of an analog splitter for splitting the high-frequency data calls from the low-frequency voice calls. Low-pass filter 86 is a network of inductors 92 in series and capacitors 94 in parallel. Since the series-inductor, parallel-capacitor network passes low-frequency signals but resist high-frequency signals, the low-frequency components from incoming telephone line 20 are passed through low-pass filter 86 to the telephone line card and eventually the telephone switch for POTS voice calls.
High-pass filter 88 is a network of capacitors 98 in series and inductors 96 in parallel. Series-capacitor, parallel-inductor networks pass high-frequency signals but block low-frequency components. Thus high-pass filter 88 passes the high-frequency components from incoming telephone line 20 while blocking the low-frequency POTS components. The high-frequency components are passed on to a data highway through an XDSL modem.
While such analog splitters are useful, they are relatively large and bulky. Inductor coils in particular are much larger than semiconductor components. The bulky coils are an impediment to miniaturization and cost reduction of ADSL. Since ADSL requires splitting the high- and low-frequency data and voice components, such bulky splitters have been considered a necessary part of ADSL. Inductors and other passive components are not easily integrated into semiconductor integrated circuits, as they cannot be mimicked by transistors.
MULTIPLE A/D CONVERTERS USED AT CENTRAL OFFICE--FIG. 4
FIG. 4 is a diagram of prior-art ADSL line equipment at a central office. Copper phone line 20 is received at the central office by analog splitter 22. Analog splitter 22 uses bulky inductor coils together with resistors and capacitors to form high-pass and low-pass filters. The output of the high-pass filter in analog splitter 22 is the high-frequency data components, which are sent to analog-digital A/D converter 24. A/D converter 24 converts the analog voltages from analog splitter 22 to digital values at a high sampling rate. These digital values are sent to ADSL processor 26, which extracts the encoded data values transmitted from the ADSL modem at the customer premises.
The data from ADSL processor 26 is combined with other such data, for transmission over a shared high speed data pathway 30. The high speed Data Network can be a fiber-optic backbone or other high-bandwidth network. ADSL processor 26 also receives data from high-speed data pathway 30 and encodes the data. The encoded data is then converted to analog voltages by A/D converter 24, and the resulting analog-voltage waveform is sent to analog splitter 22 where it is combined with the low-frequency voice signal and transmitted to the customer's premises over phone line 20. The low-frequency components output from the low-pass filter in analog splitter 22 are sent to conventional telephone line card 28, which is a standard line card used to terminate POTS phone lines. The analog signals from conventional telephone line card 28 are converted to digital values and encoded as PCM signals by A/D CODEC 32. The PCM signals from A/D CODEC 32 are combined with signals from other line cards (not shown) and transmitted over PCM highway 34 to other central offices, or to a long-distance network.
Incoming voice calls from PCM highway 34 are decoded and converted to analogvoltage waveforms by A/D CODEC 32. The analog-voltage waveforms are driven by conventional telephone line card 28 and then mixed with the high-speed ADSL data by analog splitter 22 and transmitted out on copper phone line 20.
The prior-art ADSL equipment at the central office is undesirable for several reasons. Splitter 22 is a bulky, expensive component using large inductor coils, capacitors, and resistors. Since splitter 22 is made of discrete components, it does not benefit from the continuing size and cost reductions of semiconductor integrated circuits.
SEGREGATED ARCHITECTURE, MULTIPLE EQUIPMENT RACKS
The prior-art ADSL architecture treats the ADSL and voice data as two completely separate data streams, each with its own encoders, decoders, and analog-digital converters. Such redundancy is wasteful. For example, FIG. 4 showed two separate points of analog-digital conversion, one for ADSL data at A/D converter 24, and another for conventional voice telephone calls at A/D CODEC 32.
Since two completely separate data streams are used, the ADSL and voice circuitry are typically segregated to different circuit boards or even separate equipment racks at the central office. For example, one rack may contain the conventional telephone line cards, while another rack contains the analog splitters and the ADSL modems. Often the racks are several feet apart. When many ADSL lines are terminated in a central office, the result can be a messy nest of wires connecting the ADSL and conventional telephone racks.
Terminating the phone line in such a central office is complicated by the multiple destinations of the signals. Instead of a single termination point on a single line card as for POTS, at least two termination points exist. The ADSL and POTS termination points can be physically several feet apart, complicating line provisioning.
What is desired is improved ADSL equipment at the central office. It is desired to eliminate the bulky analog splitter with its inductor coils and discrete components. It is further desired to reduce redundancy at the central office. It is desired to use a single A/D converter and to combine the encoding and decoding of POTS voice and ADSL data signals. It is desired to reduce wiring between racks at the central office by using a single line card for both ADSL and POTS signals from an ADSL line. It is desired to have a single termination point for the copper-pair ADSL phone line.